Cermets are composite materials comprised of a hard component, which may or may not be interconnected three dimensionally, and a binder that ties together or binds the hard component. An example of a traditional cermet is a tungsten carbide (WC) cermet (WC-cermet), also known as cobalt cemented tungsten carbide and WC--Co. Here, the hard component is WC while the binder is cobalt (Co-binder) as, for example, a cobalt-tungsten-carbon alloy. This Co-binder is about 98 weight percent (wt. %) cobalt.
Cobalt is the major binder for cermets. For example, about 15 percent of the world's annual primary cobalt market is used in the manufacture of hard materials including WC-cermets. About 26 percent of the world's annual primary cobalt market is used in the manufacture of superalloys developed for advanced aircraft turbine engines-a factor contributing to cobalt being designated a strategic material. Up to about 45 percent of the world's primary cobalt production is located in politically unstable regions. These factors not only contribute to the high cost of cobalt but also explain cobalt's erratic cost fluctuations. Therefore, it would be desirable to reduce the amount of cobalt used as binder in cermets.
Prakash et al. attempted to achieve this goal in their work relating to WC-cermets by substituting an iron rich iron-cobalt-nickel binder (Fe--Co--Ni-binder) for the Co-binder. (see e.g., L. J. Prakash, Doctoral Thesis, Kernforschungszentrum Karlsruhe, Germany, Institute Fuer Material-und Festkoeperforschung, 1980 and L. J. Prakash et. al., "The Influence Of The Binder Composition On The Properties Of WC--Fe/Co/Ni Cemented Carbides" Mod. Dev. Powder Metall (1981), 14, 255-268). According to Prakash et al., WC-cermets having an iron rich Fe--Co--Ni-binder were strengthened by stabilizing a body centered cubic (bcc) structure in the Fe--Co--Ni-binder. This bcc structure was achieved by a martensitic transformation.
Guilemany et al. studied the mechanical properties of WC-cermets having a Co-binder and enhanced corrosion resistant WC-cermets having a nickel rich nickel-iron substituted Co-binder at high binder contents made by sintering followed by HIPping. (see e.g., Guilemany et al., "Mechanical-Property Relationships of Co/WC and Co--Ni--Fe/WC Hard Metal Alloys," Int. J. of Refractory & Hard Materials (1993-1994) 12, 199-206).
Metallurgically, cobalt is interesting since it is allotropic--that is, at temperatures greater than about 417.degree. C., pure cobalt's atoms are arranged in a face centered cubic (fcc) structure and at temperatures less than about 417.degree. C., pure cobalt's atoms are arranged in a hexagonal close packed (hcp) structure. Thus, at about 417.degree. C., pure cobalt exhibits an allotropic transformation, i.e., the fcc structure changes to the hcp structure (fcc.fwdarw.hcp transformation). Alloying cobalt may temporarily suppress the fcc.fwdarw.hcp transformation stabilizing the fcc structure. For example, it is known that alloying cobalt with tungsten and carbon to form a Co--W--C alloy (Co-binder) temporarily stabilizes the fcc structure. (See e.g., W. Dawihl et al., Kobalt 22 (1964) 16). It is well known however, that subjecting a Co--W--C alloy (Co-binder) to stress and/or strain induces the fcc.fwdarw.hcp transformation. (See e.g., U. Schleinkofer et al., Materials Science and Engineering A194 (1995) 1 and Materials Science and Engineering A194 (1996) 103) In WC-cermets having a Co-binder the stress and/or strain developed during the cooling of the cermets following densification (e.g., vacuum sintering, pressure sintering, hot isostatic pressing . . . etc.) may induce the fcc.fwdarw.hcp transformation. Also, it is well know that cyclic loading, such as cyclic loading that may propagate subcritical crack growth, of WC-cermets having a Co-binder induces the fcc.fwdarw.hcp transformation. Applicants have determined that in cermets the presence of the hcp structure in the binder can be detrimental since this can result in the embrittlement of the binder. Thus, it would be desirable to find a binder that not only provides cost savings and cost predictability but also does not exhibit embrittlement mechanisms such as local fcc.fwdarw.hcp transformations.
For the foregoing reasons, there is a need for a cermet having a binder with higher plasticity compared to the Co-binder that can be inexpensively manufactured.